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How a Toilet Works

The simple but ingenious mechanics of the toilet have changed very little since the earliest “water closet” was invented by Thomas Crapper in the nineteenth century.

The toilet, though not one of the more glamourous of home fixtures, is designed to do a very specific job—to carry away waste and prevent sewer gasses from entering the house. And unless something goes wrong with a toilet, it handles its job efficiently.

As shown in the drawings below, a toilet has two main parts, both made from vitreous china: a tank and a bowl. Some toilets are cast as a single piece others are made in two separate parts that are joined together.

The Tank

The tank, which houses all the working parts, is where various types of toilets differ the most. Several different types of mechanisms are used to accomplish a toilet’s basic operation. Here is how the most common mechanism works:

When a toilet is ready for use, both tank and bowl are partly filled with water. Passages between the bowl and the closet bend (the top of the waste pipe) form a trap that remains filled with water at all times, blocking the rise of sewer gasses.

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The Trip Lever

When you flush the trip lever, it lifts a rubber stopper—called a tank ball, flush valve seat ball, or the newer, more effective, flapper or flapper ball—from the flush valve, letting the water in the tank flow into the bowl.

The pressure of the cascading water forces the bowl’s water and waste down the waste pipe. The water flowing into the bowl also cleans the bowl. The bowl’s water is replenished by water entering from the tank through a refill tube.

The Flush Valve

The flush valve and the flapper together are called—not surprisingly—the flapper valve. Most flush valves are 2 1/2 inches in diameter, although some of the newer models can be as large as 4 inches, as is the ball-shaped part of the flapper. The flapper hinges onto the vertical overflow pipe that’s next to the valve, and a small chain connects the flapper to the trip lever.

The advantage of a flapper over the earlier stoppers is that it doesn’t have as many parts to foul or get hung up, so it’s less likely to let the tank “run” or leak into the bowl.

As the tank of a conventional toilet empties, a float ball drops, activating the ballcock (simply a water valve), which releases water into the tank. (Some new ballcocks operate on water pressure—they don’t have a float ball.)

The water is delivered to the ballcock through a supply tube that’s connected to a valve at the wall or floor. When turned clockwise, this valve shuts off the flow of water to the tank. To prevent overflow and flooding, the top of the overflow tube is open and acts as a drain if the tank’s water level rises too high.

Modern Toilets

In today’s water-conserving models, a minimum-flush mechanism seals the flush valve seat when the tank is still partially full, keeping full pressure on the flush but using less water. A pressurized cylinder inside the toilet tank cuts water usage by putting a small amount of flush water under pressure—either from compressed air or from the house supply line’s water pressure. A pressure-activated ballcock is activated by a drop in the tank’s water pressure. This type, easily adjusted to deliver various amounts of water to the tank, eliminates the need for a float.

The conventional float-ball, lift-wire, and tank-ball mechanism has been the standard flushing device for many years. The cut-away view shows the relationship of the tank to the bowl and how the toilet’s base forms a trap to block sewer gasses. The flush handle raises the trip lever, raising the flush valve or seat ball from the flush valve seat, letting water rush into the bowl.

The stop valve at the wall delivers water through a supply tube to the ballcock. When the float ball drops, the ballcock opens, filling the tank until the ball floats back to its upper position. The overflow tube sends excess tank water to the bowl. The refill tube replenishes water in the tank through the overflow tube.